Francisella tularensis is the causitive agent of tularemia, a severe zoonotic disease in humans with fatality rates exceeding 30% in untreated subjects. Infection following inhalation causes the most severe form of disease and combind with its high virulence and history of weaponization makes this pathogen a concern for use as a biological weapon. Development of a vaccine that protects against aerosol infection is a priority. We have engineered a series of live attenuated vaccine candidates based on the highly virulent Schu S4 strain that exhibit a range of protective capacities in the mouse and rabbit models of respiratory challenge. Our studies have confirmed that rabbits are an excellent model of human pneumonic tularemia and a highly relevant model for evaluating tularemia vaccines as they display similar resistance to attenuated strains and susceptibility to virulent strains as humans. We hypothesize that the outbred human-relevant, rabbit model will allow us to identify correlates of protection using the collection of vaccine strains that confer a range (0% to 83%) of protective efficacies against lethal aerosol challenge. This grant seeks to understand the mechanism(s) underlying this protection. Our hypothesis is that effective vaccination in the rabbit model is a product of i) induction of particular, Ag-specific responses and/or ii) the immunogen's persistence and immuno-stimulatory properties. The first aim will identify the bacterial Ag(s) recognized specifically by protective immune responses. The second aim will explore the humoral and cellular immune response as well as the role of persistence and immune stimulation by F. tularensis derivatives in protection against challenge. The third aim will evaluate the efficacy of a subunit vaccine and an optimized live attenuated vaccine against inhalation of F. tularensis. The information gained under this proposal will expand our understanding of the humoral and cellular immune response to tularemia vaccines, the importance of these responses in protection against tularemia in the rabbit and the relevance to protection in humans.